1. Field of the Invention
The present invention relates to an optical fiber composite in which two types of optical fibers having a chromatic dispersion opposite in sign to each other at the wavelength of a signal-carrying lightwave are spliced in cascade, an optical fiber cable comprising a plurality of the optical fiber composites, and methods for producing the optical fiber composite and the optical fiber cable.
2. Description of the Background Art
Optical wavelength division multiplexing (WDM) transmission systems enable large-capacity optical communication by using a plurality of wavelengths. Optical fiber transmission lines to be installed in the repeater sections of a WDM transmission system are required to have an excellent transmission property in the wavelength band of the signal-carrying lightwave, for example, a 1.55 μm-wavelength band. In order to meet this requirement, researchers and engineers have proposed optical fiber transmission lines having an ununiform transmission property along the length of the transmission line.
For example, a first literature (T. Naito, et al., “1 Terabit/s WDM Transmission over 10,000 km,” ECOC '99, PD2-1, 1999) states an optical fiber transmission line consisting of the first optical fiber placed at the upstream side and the second optical fiber placed at the downstream side in terms of the propagation direction of a signal. The first optical fiber has a positive chromatic dispersion and a relatively large mode-field diameter at a 1.55-μm-wavelength band. The second optical fiber, a dispersion-compensating optical fiber (DCF), has a negative chromatic dispersion and a relatively small mode-field diameter at a 1.55-μm-wavelength band.
With this optical fiber transmission line, although a signal-carrying lightwave is intense when it starts to propagate in the first optical fiber, nonlinear optical phenomena seldom occur because the first optical fiber has a relatively large mode-field diameter. The lightwave decreases its intensity during the propagation through the first optical fiber. Because the lightwave with decreased intensity propagates through the second optical fiber, nonlinear optical phenomena seldom occur despite the second optical fiber having a relatively small mode-field diameter. In addition, because the two optical fibers have a chromatic dispersion opposite in sign to each other, a proper selection of the ratio of the lengths between the two optical fibers enables the overall chromatic dispersion of the entire optical fiber transmission line to be nearly zero. As described above, signal degrading due to the generation of nonlinear optical phenomena and a large value of overall chromatic dispersion can be avoided by using an optical fiber transmission line in which the first optical fiber and the second optical fiber are spliced in cascade in terms of the propagation direction of a signal-carrying lightwave.
A second literature, U.S. Pat. No. 5,894,537 entitled “Dispersion Managed Optical Waveguide,” discloses an optical fiber transmission line in which a section having a positive chromatic dispersion at the wavelength band of a signal-carrying lightwave and another section having a negative chromatic dispersion at the same wavelength band are provided alternately and successively in the longitudinal direction. Because both sections are designed to have a large absolute value in chromatic dispersion, the occurrence of nonlinear optical phenomena such as four-wave mixing can be suppressed. Furthermore, because the overall chromatic dispersion of the entire optical fiber transmission line is designed to be small in absolute value, the degradation in the transmission quality can be suppressed.